Tumor cells traverse epithelium basement membranes and collagenous stroma during the transition from in situ to invasive carcinoma and during passage through blood vessel walls and tissue boundaries in the metastatic process. In vivo studies have shown that circulating tumor cells first attach to the exposed basement membrane. Attachment is followed by focal dissolution and penetration by tumor cell pseudopodia. No previous in vitro system has existed for studying this process at the biochemical or ultra structural level. We therefore have prepared whole sheets of intact human amnion basement membrane in an in vitro model of tumor cell invasion. The amnion is placed in a special chamber which allows cells to be placed on one side. Cells which have traversed the amnion are collected and counted on a millipore filter.